1. Field of the Invention
This invention relates to the fabrication of thin-film optical devices and, more particularly, to the fabrication of large numbers of such optical devices in a low-stress configuration.
2. Description of the Related Art
Thin-film optical devices, such as thin-film optical filters, are used to separate light of different wavelengths. The thin-film optical filters are formed as a series of thin layers of material deposited one over the other, with as many as several hundred layers in total. Light of some wavelengths is transmitted through the thin-film optical filter, while light of other wavelengths is reflected from the thin-film optical filter. The specific transmitted and reflected wavelengths are determined by the selection of the materials of construction and the thicknesses of the layers. The thin-film optical filters are included in optical systems to perform any of a variety of functions that require separation of a light beam into its component wavelengths. An example of such an optical system is a wavelength division demultiplexer.
To fabricate large numbers of thin-film optical filters by conventional mass-production procedures, a transparent substrate of the desired final thickness is provided. The multiple layers that form the thin-film-optical filter are deposited onto the substrate. The substrate and the overlying thin film structure are thereafter diced into smaller pieces that constitute the individual thin-film optical filters.
As part of the work leading to the present invention, the inventors have recognized that the thin-film optical devices produced by this approach have several shortcomings. Foremost among these shortcomings is the residual stresses that result from the elevated-temperature deposition of layers of differing coefficients of thermal expansion onto a substrate with yet a different coefficient of thermal expansion. Upon cooling and dicing, the thin-film optical filters bend responsive to the residual differential thermal strains, warping the thin-film optical structure so that a restraining structure is needed to hold the thin-film optical structures in a flat configuration. Additionally, the edge of each layer of the thin-film optical structure is subject to corrosion and other degradation during use.
There is therefore a need for an approach to the fabrication of thin-film optical devices that overcomes these problems. The present invention fulfills this need, and further provides related advantages.
The present invention provides a method for fabricating thin-film optical devices that greatly reduces the internal stresses within the optical device, and therefore the tendency for the optical device to warp, as compared with conventional approaches. The edges of the thin films are sealed in the fabrication process, reducing their susceptibility to corrosion. The present approach does not require any change in the nature or thicknesses of the thin films that make up the optical device, so that there is no change to the functionality of the thin-film optical device. The present approach is applicable to a wide range of thin-film optical devices, such as thin-film optical filters.
In accordance with the invention, a method for fabricating thin-film optical devices, such as thin-film optical filters comprises the steps of providing a substrate having a first side and a second side, thereafter forming a pattern of channels on the first side of the substrate to define a plurality of mesas on the first side of the substrate, thereafter depositing a thin-film optical structure onto each of the mesas, and thereafter removing an excess portion of the substrate. The thin-film optical structure is transparent to a selected wavelength of light, and the substrate is preferably transparent to that same selected wavelength of light.
The step of removing is preferably performed by temporarily affixing a temporary support to the top of each thin-film optical structure, removing an excess thickness of the substrate from the second side of the substrate so that the thin-film optical structures are isolated from each other but affixed to the temporary support, and separating the tops of the thin-film optical structures from the temporary support. The step of temporarily affixing may include utilizing a non-permanent adhesive to affix the temporary support to the tops of the thin-film optical structures, and the step of separating includes the step of debonding the non-permanent adhesive by an appropriate technique such as a solvent or heating.
The pattern of channels may be produced by any operable technique, with sawing being preferred. The thin-film optical structure may be of any operable type. The deposition preferably is performed by mounting the substrate on a rotating deposition stage such as a planetary deposition stage, and depositing the thin-film optical structure from a deposition source while rotating the substrate on the rotating deposition stage. This rotational deposition results in the deposition of the material of each layer onto the layer, but onto the sides of the optical device as well, thereby sealing the edges of the previously deposited layers.
The present approach cuts the pattern of channels to define the mesas prior to the deposition of the thin-film layers. As a result, the residual stresses that are created during the deposition process are limited by the relatively short lengths of each mesa that lie in the plane of the deposited layers. By contrast, in the conventional process that dices the substrate and deposited layers after deposition of the thin-film structure, the residual stresses increase proportional to the total lateral dimension of the substrate and therefore are much larger than experienced with the present approach. The result is that there is far less tendency in the present approach for the individual thin-film optical devices to warp responsive to the residual stresses, as compared with the prior approach. The present approach also seals the edges of the deposited layers with the material of each successively deposited layer, so that the edges of the deposited layers are not exposed to corrosion during their later service.